Populations of O. orestes are apparently confined to open habitats above treeline and exhibit a characteristic morphology: medium-sized species with relatively short tail; fur very deep, soft, and dense with creamy-buff post-auricular patches present; cranial arching strongly pronounced, anterior zygomatic arches squared, and distal expansion of nasals less exaggerated; M3 laminae 6-7, lower incisors with deep lateral and shallow medial grooves. Thomas (1900b) appreciated such morphological distinctions between O. orestes and samples from the middle slopes of Mount Kenya that he later (1902c) recognized as O. tropicalis, as did other early workers with East African Otomys (Wroughton, 1906; Dollman, 1915; Hollister, 1919; Lawrence and Loveridge, 1953). Bohmann (1952) also considered the two Mount Kenyan series as separate species, although he viewed orestes as a synonym of a broadly, if patchily, distributed O. typus (as followed by Misonne, 1974; Musser and Carleton, 1993; and others). M3 lamination (8-9 in typus) and incisor corrugation (two deep grooves in typus) contradict this assignment and recommend rigorous evaluation of their level of relationship and taxonomic status. Similar attention should be devoted to the homogeneity of the included taxa, which vary in development of the second lower incisor groove (indistinct to moderate) and modal number of M3 laminae (6 or 7).